Experimental Study of Strain Rate Sensitivity to Fracture Toughness of Rock using Flattened Brazilian Disc

Brazilian splitting test with arc loading condition may result in a reduction of the concentration of stress in the disc. Additionally, crack initiation from the central position of the Brazilian disc cannot be guaranteed because of the non-uniformity of stress-loading arcs. In order to achieve the theoretical foundation of the Brazilian splitting test, a Flattened Brazilian Disc (FBD) specimen can be used as an alternative, which prevents local crack initiation. An FBD specimen, because of its ease of sample preparation and loading geometry, can be used to estimate different geomechanical properties of rocks such as tensile strength (σt) and mode I fracture toughness (KIC). To validate the indirect tensile test by using the complete Brazilian disc, the crack must be initiated from the central region of the specimen, and the loading angle that corresponds to the arc of the disc must be greater than 19.5°. In the current study, an attempt has been made to investigate the sensitivity of the strain rate to various geomechanical properties (tensile strength and fracture toughness) of a homogeneous fine-grained sandstone by using FBD specimens. The strain rates were varied between low and intermediate rates in the range of 10-5 to 10-2 s-1. Two sets of specimens were tested with a loading angle of 2α = 20° and 2α = 30° in order to ensure better experimental observation of the strain rate sensitivity to various geomechanical properties. The test results indicate that with increasing strain rates, the fracture toughness, and tensile strength gradually increase. The fracture toughness that corresponds to a loading angle of 20° overestimates the fracture toughness of the rock.